1. Field of the Invention
This invention deals with the techniques of bending glass sheets and, more especially, with the devices and processes forming part of these techniques, which enable the glass sheets to be transferred and/or supported inside the shaping station.
2. Description of Related Art
These techniques are intended, preferably, for the production of panes for automobiles, whether they be bent and then toughened (most commonly used for the side and rear windows of the vehicle) or bent and then annealed for the purpose of being assembled to form laminated panes comprising, for example, two glass sheets (used preferably as windscreens).
The panes, especially for automobiles, must comply with very severe criteria, preferably with regard to optical quality and conformity to the prescribed curvature. This is why every effort is made to minimize the risks of defects, for example by friction of the glass sheets against the tools and molds with which they are in contact, thereby creating marks on their surface, and/or as a result of incorrect positioning of the sheets relative to said tools and molds, preferably relative to the bending molds, thereby leading to non-conforming curvatures. This aspect is especially crucial when the sheets are situated in the shaping station, having reached their softening temperature, and are undergoing bending by suction and/or mechanical pressure in direct contact with bending molds.
It may be recalled that in certain conventional bending techniques the glass sheets travel horizontally to the reheating furnace, conveyed on a bed of rollers which continues right into the shaping station, from which roller bed they are then raised, preferably by peripheral suction created by a suction box, into contact with the lower, curved face of an upper bending mold, the curvature of which face they hug more or less closely under the effect of the suction. This shaping may then be completed, either under the combined action of inertia and gravity by dropping the sheets onto an annular lower mold, or by mechanical pressing of the sheets between upper mold and annular lower mold. These sheets are then removed, either on the same lower annular mold or on a special annular mold for toughening, to the toughening/ thermal treatment station. Reference may be made to Patents FR-B-2 085 464; EP-B-241 355; EP-A-240 418; EP-A-241 355; EP-A-255 432 and EP-A-389 315.
A problem arises, however, in choosing the means for transferring and/or supporting the glass sheets in this station, which means should be the most suitable for maintaining the optical quality of said glass sheets at high temperatures.
The aforementioned bending techniques suggest, in fact, as a means for transferring the glass sheets from the furnace to the shaping station and then as a means for supporting the glass sheets in this station before they are taken over by the bending molds, a simple succession of conveying rollers continuing the roller bed that is present in the furnace. This implies a direct mechanical contact between rollers and softened sheets in the station, which limits the possibilities of recentering the sheets relative to the bending molds (a recentering carried out, for example, by the means explained in Patent Application EP-A-389 315, already mentioned), since any repositioning of the sheets leads to a relative sliding between sheets and rollers, and the resulting friction which generates marks.
Other bending techniques have proposed different means for conveying and/or supporting the sheets in the shaping station. Patent Appl. EP-A-0 351 278 describes a bending process consisting of conveying the sheets from the furnace to the shaping station by means of a flexible conveyer belt bringing the sheets to a stop underneath the bending mold which, by suction, then takes them over. This belt may, in addition, participate in the bending by fulfilling the function of a lower, pressing counter-mold. The point contacts between rollers and sheets are thus replaced here by a continuous contact between sheets and flexible material, but this is still a mechanical contact that is liable to create marks on the sheets.
There is also known an "entire air cushion" process, described preferably in U.S. Pat. No. 3,869,271, in which successive air cushions in the furnace continue into an air cushion in the shaping cell, thus providing for the entirety of the conveying of the sheets. The performance of this process, however, is difficult, because it is not easy to guarantee good planarity of the entirety of the cushions over such a considerable length, more especially since the sole embodiment uses pressurized boxes of porous ceramic, which are very difficult to manufacture and expensive and are variable in their reproducibility.
Furthermore, the use of an air cushion in the upstream part of the furnace, that is when the glass sheets are still very stiff, tends to create defects in planarity on these sheets, preferably transverse concavities relative to their axis of travel, which defects can lead to accidental physical contacts between the lower face of the glass sheets and the box generating the air cushion.